In the sewn goods industry, where various sections of material are sewn together to fabricate products, precise seam lengths and end points are often necessary for proper appearance and function of the finished products. For example, the top stitch seam of a shirt collar must closely follow the contour of the collar and terminate at a precise point which matches the opposite collar. Accurate seam lengths must similarly be maintained in the construction of shoes when sewing together vamps and quarter pieces to achieve strength as well as pleasing appearance. Achieving consistently accurate seam lengths and end points at high rates of production has, however, been a long standing problem in the industry.
Microprocessor controllers have been developed which convert manually operated sewing machines into semi-automatic sewing systems. U.S. Pat. Nos. 4,108,090; 4,104,976; 4,100,865; and 4,092,937, assigned to the Singer Company are representative of such devices. Each of those patents discloses a programmable sewing machine with three operational modes: manual, teach and auto. Control parameters are programmed into the system for subsequent control of the sewing machine in the auto mode. Those microprocessors control all sewing machine functions such as sewing speed, presser foot position, thread trimmer, reverse sew mechanism and the number of stitches sewn in each individual seam. Accurate control of seam lengths is one of the important aspects of those systems.
U.S. Pat. No. 4,404,919 issued Sept. 20, 1983, entitled "Control System for Providing Stitch Length Control of a Sewing Machine", assigned to assignee describes a microprocessor controlled sewing system which improves upon the seam length accuracy of those systems. The system disclosed in U.S. Pat. No. 4,404,919 controls seam length accuracy using a combination of stitch counting, edge detection and stitch length control techniques. Control of seam lengths and end points is achieved in the system by initiating countdown of a variable number of final whole and partial stitches responsive to detection of the end of the material being sewn by sensors located ahead of the needle. In dependence upon the amount of the stitch which has been sewn upon edge detection, the microprocessor issues a signal to position the reverse sew mechanism of the sewing machine while the last stitch is being formed to reduce the length of the last stitch to a desired percentage of the normal stitch length and thus improve the accuracy of the seam end point.
Though ideally the time delay between the microprocessor issuing the signal to activate the reverse sew mechanism and the actual movement of the mechanism to reduce the length of the last stitch is zero, in practice, that time delay is typically in the range of 10 to 40 milliseconds. Were the sewing machine operated so that stitches are formed continuously during a complete revolution of the sewing machine motor, that delay could be easily compensated for and the desired results achieved be issuing the signal to activate the reverse sew mechanism, for example, 10 to 40 milliseconds early. However, the formation of stitches in a typical sewing machine occurs in an intermittent manner, each stitch being formed during approximately 120 degrees of revolution for each complete revolution of the motor and no stitch formation occurring in the remaining 240 degrees of revolution. The combination of retraction time delay and intermittent feed often causes the length of the last stitch to vary from the desired length.
A need has arisen, therefore, for an improved adaptive sewing machine control system which includes a stitch length control technique which compensates for the activation time delay of the reverse mechanism and intermittent feed characteristics of the sewing machine to accurately reduce the length of the last stitch.